1. Field of the Invention
The present invention relates to a technique concerning a method of manufacturing a semiconductor device using a semiconductor film, and particularly to a method of manufacturing a semiconductor device including a thin film transistor (hereinafter abbreviated to TFT) using a crystalline silicon film.
Incidentally, in the present specification, the semiconductor device indicates any device functioning with semiconductor, and includes not only a single component such as a TFT but also an electro-optical device, a semiconductor circuit, and an electronic equipment incorporating those.
2. Description of Related Art
In recent years, a TFT used for an electro-optical device such as an active matrix type liquid crystal display device has been actively developed.
With respect to the active matrix type liquid crystal display device, a monolithic type display device in which a liquid crystal display portion and a driver circuit portion are provided on the same substrate becomes the mainstream. Moreover, a system-on-panel having a built-in logic circuit portion, such as a gamma correcting circuit, a memory circuit, and a clock generating circuit, has been developed. Since such a driver circuit portion and a logic circuit portion require a high mobility, a TFT using a crystalline silicon film (polysilicon film) is used. When the crystal grain diameter of the crystalline silicon film is large, a TFT with an extremely high mobility can be obtained.
The TFT using the crystalline silicon film is generally a coplanar type, and a semiconductor technique such as thermal oxidation or ion implantation can be applied. It is considered that especially when a thermal oxidation step is used, defects of a silicon film are compensated, so that TFT characteristics such as mobility, subthreshold coefficient, and threshold voltage are improved, and an extremely excellent switching characteristic and a high speed operation characteristic can be realized.
A mechanism of defect compensation by thermal oxidation has been considered as follows. In thermal oxidation of a silicon film, oxygen diffuses in a thermal oxidation film (silicon oxide film) and reaches an interface of SiO.sub.2 /Si, where an oxidation reaction occurs so that a new thermal oxidation film is formed. When silicon is oxidized to become silicon oxide, about double expansion by volume occurs. For the purpose of causing oxidation to progress in a closed state of the interface, it is necessary to secure a space for volume expansion. Surplus silicon -atoms diffuse, as inter-lattice silicon atoms, through a silicon oxide film or silicon film in just the state they are unreacted. It is considered that the reason why the characteristics of a TFT using a crystalline silicon film is improved by a thermal oxidation step, is that the inter-lattice silicon atoms are supplied to lattice defects, such as grain boundaries of the crystalline silicon film or defects in grains, so that the defects are compensated. Since a crystalline silicon film in which defects are compensated can be formed by the thermal oxidation step, the thermal oxidation step is a very useful technique in manufacture of a TFT with a high mobility.
However, when a crystalline silicon film 301 shown in FIG. 2A is thermally oxidized, roughness is formed on the interface between a thermal oxidation film 302 and a crystalline silicon film 303 or on the surface of the thermal oxidation film 302 as shown in FIG. 2B, and the thickness of the crystalline silicon film 303 used as an active layer becomes irregular, so that there rises such a problem that fluctuation in TFT characteristics occurs in the same substrate. Thus, the thermal oxidation step cannot be directly used. Since the thermal oxidation film is a dense film, its insulating property is excellent and it is a film suitable for a gate insulating film. However, when the thermal oxidation film 302 obtained by thermal oxidation of the crystalline silicon film is used as the gate insulating film, since the film thickness does not become uniform by the formation of roughness, a gate voltage is concentrated on a thin portion of the film thickness so that it may cause breakdown.
Formation of the roughness of the thermal oxidation film is caused by the fact that the oxidation speed of the crystalline silicon film is different in different places. The oxidation speed is different by the difference in plane orientation of each crystal, and is also different between a crystal and a crystal grain boundary. Especially, as compared with a crystal, an oxidation speed at a crystal grain boundary is fast, and oxidation progresses selectively from the crystal grain boundary, which contributes largely to the formation of roughness. Especially, when crystals with large grain diameters are thermally oxidized, irregular roughness is formed so that it has not been preferable. However, as described above, in order to realize a TFT with a high mobility, it is preferable to increase the crystal grain diameter of a crystalline silicon film used as an active layer. Thus, such a countermeasure has been adopted that an active layer is not directly thermally oxidized, but a silicon film with crystal grains of uniform small diameters is formed on a crystalline silicon film used as an active layer, and this silicon film is thermally oxidized.
Like this, when the crystalline silicon film with a uniform small crystal grain diameter is formed on the crystalline silicon film used as the active layer and the crystalline silicon film with the small grain diameter is thermally oxidized, the roughness of the thermal oxidation film can be certainly lessened. However, if crystals with large grain diameters are included in the silicon film to be thermally oxidized, even if the number of the crystals is one, the film thickness of the active layer thereunder becomes irregular, and fluctuation of TFT characteristics occurs in the same substrate, so that it is not preferable. In order to solve this problem, strict control of the crystal grain diameter has been indispensable. As a method of strictly controlling the grain diameter, a method of carrying out crystallization at the same time as film formation is superior in that a film formation speed and temperature can be controlled at the film formation. Although it has been possible to make crystals have uniform grain diameters of 100 nm or less by using this method and by strictly controlling conditions, the control conditions are severe and cannot be easily attained. Besides, for the purpose of meeting a request for high speed operation, the film thickness of an active layer or a gate insulating film has been decreased, and crystals have been required to have uniform smaller grain diameters.
In the first place, if only an amorphous silicon film can be directly thermally oxidized, the thermal oxidation step can be used without any problem. However, it has been considered to be difficult. This is because thermal oxidation of an amorphous silicon film generally requires a temperature of 800.degree. C. or more, and crystallization starts at about a temperature of 600.degree. C. in the process of a temperature rise. Further, in the thus crystallized silicon film, it is difficult to control nucleus production, crystal grain diameter, or the like in the crystallizing step, so that the silicon film comes to have random and irregular crystals. When this film is thermally oxidized, irregular roughness is formed on a thermal oxidation film, so that it has not been preferable.